Exercise constitutes a substantial stress, and when performed in a hot environmental thermal injury and mortality are exacerbated. The ability of an organism to withstand heat stress is termed thermal tolerance. While the mechanism of thermal tolerance in animals is unknown, at a cellular level, thermal tolerance is associated with the synthesis of heat shock or stress proteins (HSPs). The purpose of the present study is to evaluate the role of HSP synthesis in the development of whole body thermal tolerance, and to determine whether these proteins can serve as biochemical markers for the development of thermal tolerance or heat injury. In preliminary studies, we have successfully developed a model which allows the quantitation of alterations in HSP synthesis in humans exercising in the heat. These studies are the first to define conditions under which HSP synthesis may be measured in humans. Using the rat heat stroke model, we will determine 1) the effects of exercise-induced heat stress compared with passive heat stress on the synthesis of HSPs in heart, brain, liver, small intestine, kidney, skeletal muscle, and blood lymphocytes of rats, 2) the dose response effects of heating rate as opposed to thermal load on HSP synthesis in select target tissues, 3) the mechanisms of these HSP alterations including HSP promoter induction, RNA transcription rates, mRNA to HSP using cell culture, and 4) whether the process of heat acclimation is associated with changes in the synthesis of HSPs in select target tissues. We hypothesize that 1) Synthesis of HSP will identify those tissues most susceptible to thermal damage during active (exercise) and passive hyperthermia, 2) For a given heat stress HSP synthesis and tissue damage will be greater during exercise hyperthermia compared with passive hyperthermia, and 3) Heat acclimated animals will synthesize less HSP than unacclimated animals as evidence of their improved thermal tolerance. This proposal is unique in that it combines animal, cell and molecular models to focus upon a common mechanism by which both whole animals and isolated cells may respond to heat stress. This integrated approach will provide information on the biochemistry and molecular biology of thermal tolerance and heat injury. Further, the proposal relies upon the prior work of the principle investigator into the cellular and molecular mechanisms of control of HSP synthesis and the co- investigator's expertise in heat injury and acclimation in animals to develop new insights into molecular mechanisms of exercise-induced heat injury and thermal tolerance.